1. Field of the Invention
This invention relates generally to a battery operated in-line audio amplifier and, more particularly, to an audio amplifier that employs high and low current modes of operation to extend battery life.
2. Description of the Related Art
Standard telephones typically do not include any provision for adjusting the volume of the conversation presented to the receiver portion of the telephone handset. Accordingly, where there is a poor connection or the user is hard of hearing, the volume of the conversation may be undesirably low. The user has little recourse other than asking the other party to speak more loudly. Therefor, it is desirable that the user have available an inexpensive in-line amplifier that is readily attached to a standard telephone and adjustable to the user's desired comfort level. In order for the in-line amplifier to be readily attachable to a standard telephone, it requires access to an independent source of electrical power. To avoid modifications of the telephone it is desirable that this independent electrical power source be in the form of a readily interchangeable battery.
However, use of a battery as the electrical power source for the in-line amplifier imposes severe limitations on the design of the in-line amplifier. For example, an operational amplifier used in the in-line amplifier typically lacks the current driving capability to properly operate the receiver of the telephone handset beyond relatively low level conversation. Therefor, an output stage, such as an emitter follower that draws current directly from the battery, is employed. High quality reproduction of the audio signal can be achieved by using a class A output stage, however, class A output stages typically suffer from low efficiency and high quiescent currents. It should be apparent that the use of a class A output stage will severely shorten the life of the battery and necessitate undesirably frequent changing of the battery.
Alternatively, class B output stages are well known for their high efficiency and low quiescent current draw. However, class B output stages are equally well known for the cross over distortion resulting from the dead band surrounding the zero voltage level. This dead band is, of course, caused by the complimentarily connected transistors in a class B output stage being simultaneously biased "off." Thus, while the battery life of such a device would be greatly enhanced over class A output stage, the audio signal presented to the receiver of the telephone handset would be unacceptably distorted.
Further, it should be appreciated that use of battery power for the in-line amplifier dictates that the battery will periodically discharge to such a low level as to be unable to operate the amplifier. At this point, the amplifier not only does not amplify the audio signal, but also blocks the audio signal from being delivered to the telephone handset. Thus, the user is presented with the alternatives of removing the in-line amplifier or replacing the battery so that the telephone is once again operable. Neither alternative is desirable. Rather, it is preferable that in-line amplifier should be capable of controllably enhancing the audio signal without undesirably blocking the audio signal.
The present invention is directed to overcoming one or more of the problems set forth above.